Digital cameras have been widely used, wherein subjects are photographed through an imaging device such as a CCD image sensor. The digital camera shoots video images, and displays a slew of video images on a liquid crystal display that serves as an electronic viewfinder. In response to a release button being pressed, the digital camera photographs a still image and records data of the photographed still image on a recording medium like a memory card. There are such digital cameras that can record video image data.
In such a digital camera, the image sensor is exposed and read out generally at a frame rate or shooting frequency of 30 Hz, i.e. 30 frames per second, in synchronism with the recording and displaying of the video images. Meanwhile, there are two kinds of frequencies of commercial power source in Japan: 60 Hz and 50 Hz. Being powered with the 60 Hz commercial power source, a fluorescent light will blink 120 times per second. With the 50 Hz commercial power source, the fluorescent light will blink 100 times per second. Under these periodically blinking fluorescent lights, especially those activated with the 50 Hz commercial power source, so-called flicker occur in photography with the digital camera. The flicker is a phenomenon that brings variations in photographic lightness, and is caused where the frame rate of the image sensor using the 50 Hz commercial power source does not have any common multiple to the blinking cycle of the fluorescent lights.
To prevent the flicker on shooting video images of low-brightness subjects which are illuminated with fluorescent lights, especially the fluorescent lights activated with the 50 Hz commercial power source, the conventional digital camera uses a charging time of the image sensor, i.e. a shutter speed of an electronic shutter, hereinafter called the electronic shutter speed, that is set at 1/50 sec. or 1/100 sec. whenever possible, while an aperture value is adjusted to obtain a proper exposure value.
As described above, unless the release button is pressed down, the digital camera shoots video images through the image sensor so as to display a slew of images of the shot subjects on the LCD. The photographer frames a photographic field while observing the images on the LCD, and presses down the release button to photograph a still image. When the release button is pressed halfway, the digital camera carries out an automatic exposure (AE) process for deciding a aperture value and an electronic shutter speed for photographing a still image, and an automatic focusing (AF) process for focusing a taking lens onto the aimed subject. After the AE and AF processes are completed, the digital camera shoots video images again through the image sensor, to display a slew of images on the LCD. Thus, the LCD serves as an electronic viewfinder.
Upon the release button being fully pressed down, the digital still camera makes a freeze process to display a still image of the currently shot subject on the LCD, and then switches over the aperture value to a value predetermined in the AE process. Thereafter the digital camera makes a preparation operation for setting up a timing generator in order to drive the image sensor at a timing and an electronic shutter speed, which are determined for still image photography, as well as for setting up the timing generator with an actuation timing of a mechanical shutter. After the completion of this preparation operation, the image sensor is exposed to photograph a still image.
In some digital camera, a timing generator for outputting a variety of drive signals for driving the image sensor is set up to raise frequency of vertical synchronizing signals when the release button is fully pressed down, wherein the vertical synchronizing signals serve as a reference for the operation timing. As being loaded with parameters for deciding driving timings of the image sensor and other components, the timing generator makes the loaded parameters effective when the vertical synchronizing signal is generated first after the loading, and drives the image sensor and other components on the basis of these parameters from a frame period after the completion of setting. Therefore, raising the frequency of the vertical synchronizing signals reduces the delay from the completion of setting with respect to the still image photography through the image sensor.
As described above, since the conventional digital cameras starts many processing upon the release button being pressed to the full, there is a certain time lag to the actual start of exposure. As a known technique for minimizing the release time lag, Japanese Laid-open Patent Application No. Hei 2-185173 suggests controlling driving the image sensor by a timing generator so as to start charging of the image sensor for photographing a frame of still image in synchronism with an external trigger signal, although the charging conventionally starts after several electronic shutter pulses are given at the start of exposure for the still image photography.
One of factors that enlarge the release time lag is an operation to change the aperture value at the moment when the release button is fully pressed down. However, in order to shoot video images at such an aperture value that is adapted to the still image photography prior to the full press of the release button, the shutter speed must be changed in accordance with varying subject brightness. So it becomes impossible to keep the electronic shutter speed at a value preventive against the flicker.
In addition to that, although the method disclosed in the above prior art is effective for reducing the release time lag, this prior method needs a specific timing generator, and cannot be embodied with a general timing generator.
Since the conventional digital camera validates content set in the timing generator in synchronism with the vertical synchronizing signal, there is a delay from the shutter release to the actual start of an exposure for photographing a still image. As described above, the delay, a factor for the release time lag, can be reduced by raising the frequency of the vertical synchronizing signal. But because there is a limit in the available range of frequency of the vertical synchronizing signal, this method cannot sufficiently reduce the delay and the release time lag.